Sayyiduna Anas bin Malik (may Allah be pleased with him) narrates that Sayyiduna Zayd bin Thabit (may Allah be pleased with him) said: "We ate the pre-dawn meal (suhur) with the Messenger of Allah (peace and blessings be upon him), and we got up only when he (peace and blessings be upon him) led the Fajr prayer." I asked: How long was that? He said: As long as it takes a person to recite fifty verses.
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 4778